Molecular resonance modulators and demodulators



Jan. 7, 1958 I c, ow s 2,819,450

MOLECULAR RESONANCE MODULATORS AND DEMOD-ULATORS Original Fild April 26,1947 FIG.

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MOLECULAR RESONANCE MODULATORS AND DEMDDULATQRS (Jharles H. Townes, NewYork, N. Y., assignor to Bell Telephone Laboratories, Incorporated, NewYork, N. Y., a corporation of New York Original application April 26,1947, Serial No. 744,236,

new Patent No. 2,707,235, dated April 26, 1956. Divided and thisapplication April 1, 1955, Serial No. 498,643

Claims. (Cl. 332-57) This application is a division of my copendingapplication Serial No. 744,23 6, filed April 26, 1947. which issued asU. S. Patent No. 2,707,235, April 26, 1955.

This invention relates to systems and methods for modulatingelectromagnetic wave energy, particularly energy in the microwave range.

An object of the invention is to provide a simple and efiicient systemfor modulating microwave energy by audio or other signaling waves toproduce amplitude modulations thereof representative of such signals.

Another object of the invention is to provide a simple and efficientsystem for demodulating microwave energy to recover the signal withwhich it has been amplitudemodulated.

In accordance with the present invention these and other objects areattained by the utilization of the natural resonance absorptioncharacteristics of gases. These resonant absorption characteristicsbecome manifest when the gas is subjected to electromagnetic fields,particularly to fields produced by waves of frequencies in the microwave and infrared regions. The nature of such action, the types of gasin which it is most evident, the effect of pressure conditions of thegas, the effect of electrostatic and magnetostatic fields, and relatedfeatures are fully discussed in the patent application previouslyidentified.

In accordance with the present invention a particular characteristic ofthe gas resonant absorption efliect is utilized to produce modulationand demodulation of microwaves. This phenomenon is the dependency of theabsorption coefiicient to electromagnetic waves of one frequency uponthe energy level of electromagnetic waves of a different frequency towhich the gas is simultaneously subjected. This effect is due to thefact that the sharp, selective absorption of electromagnetic wave energyof one frequency by such a gas is dependent upon the numbers of the gasmolecules that are at the various possible energy levels. The molecularpopulation at these energy levels can be varied by subjecting the gas toelectromagnetic wave energy of a frequency different from that of theenergy being absorbed. This energy varies the number of molecules in thevarious energy states, consequently varying the absorption due tomolecular resonance of the electromagnetic energy of the first frequencybeing transmitted through the gas.

in accordance with one feature of this invention microwaves which aredesired to be modulated are transmitted through a confined body of gasexhibiting molecular resonant absorption at the frequency of themicrowaves. Simultaneously the gas is irradiated by otherelectromagnetic wave energy such as infrared rays. By varying, inaccordance with a signal to be transmitted, the level of the energy ofthese other electromagnetic waves which are permitted to irradiate thegas, it is possible to vary the absorption coeflicient of the gas to themicrowaves and consequently to amplitude-modulate the microwaves takenout of the gas in accordance with such a signal.

In accordance with another feature of the invention ZQlQASQ PatentedJan. 7, 1958 microwaves that are modulated in amplitude in accordancewith a signal are caused to impinge on a gas which exhibits molecularresonant absorption with respect to other electromagnetic waves that aretransmitted therethrough. As a result the signal modulations aretransferred to these other electromagnetic waves from which they can beseparated by conventional detectors.

Referring to the figures of the drawing:

Fig. 1 shows an amplitude modulator of microwaves utilizing themolecular resonance absorption of a gas irradiated by an auxiliaryinfrared source; and

Fig. 2 shows a corresponding demodulator.

The absorption coefficient of many molecularly resonant gases formicrowaves or the like is dependent on the presence or absence of anauxiliary electromagnetic field of a different frequency. This permitsthe energy absorbed from incident electromagnetic waves to be varied atwill by subjecting the gas at low pressure to the irradiation byelectromagnetic energy of a different frequency.

In accordance with the present invention, amplitude modulation may beimparted to a microwave frequency or frequencies by passing themicrowaves through a molecularly resonant gas and irradiating the gas bya source of different frequency, such as other microwaves, light, orinfrared radiations, and varying the irradiation of the gas therebyunder the control of a desired modulating signal.

The absorption by a molecularly resonant gas of microwave energy may bemodified in amount by irradiating the gas with electromagnetic waves ofa different frequency. Thus, for example, the microwave absorption atlow pressures by ordinary ammonia gas of the molecular resonance line at23,870 megacycles is about 0.2 decibel per foot of length of the gaschamber, for a single energy pass. When, however, the gas is irradiatedby infrared radiation, the absorption of the ammonia resonance line isconsiderably reduced. In the case of other gases or electromagneticwaves of other frequencies the effect may be an increase of absorption.Whether it be a reduction or an increase depends in a complicatedfashion on the various characteristics of the particular molecule inquestion and the frequency of the disturbing radiation. In general,however, there can be a marked alteration in the absorption ofmicrowaves of a first frequency by molecular resonance when the gasmolecules are irradiated by a different frequency radiation.

This phenomenon is turned to account here to provide amplitudemodulation of microwave energy. Various methods and apparatuses may beemployed to produce such modulation and demodulation of microwaves.Apparatus of one such form is depicted in Fig. 1, wherein a microwaveoscillation source lit), which may be similar to that described inconnection with my aforementioned application or of any desired variety,delivers energy from its resonant cavity 1'7 to a gas-containingmolecular absorption cell by way of a wave guide 22. The input frequencyto the cell 17% should be substantially equal to the frequency at whichthe gas is resonant; e. g., if the gas is ordinary ammonia, thefrequency should be 24,000 megacycles (more precisely, 23,870megacycles) and the oscillation source in should be tuned to thisfrequency and stabilized thereat.

The resonant gas cell 170 may be provided with a mica window 24 at thepoint at which the input wave guide 22 is coupled to it and with awindow 25 at the point to which an output wave guide 26, leading to anamplifier 171 and a load circuit, e. g., an antenna 172, is coupled toit. The gas may be introduced by way of a valve 32 and its pressureadjusted by means of a pump 33. The cell 170 may be a wave guide,preferably of the resonant type hereinabove discussed. Its dimensionsshould be selected so that its resonant frequency, regarded as a tunedcavity, coincides with the resonant frequency of the gas, and its tuningshould preferably be substantially broader than the resonant absorptionband of the gas.

An infrared radiation source 174, for example, an electrically heatedplatinum ribbon or a caesium vapor lamp, is provided, whose rays 175 maybe reflected on a suitable mirror 176, for example a polished coppersurface, to enter the resonant gas chamber 170 by way of a window 177which is transparent to infrared radiation. of the frequency inquestion, for example a window of silver chloride. The mirror may becaused to vibrate in accordance with a signal, being mounted, forexample, in the familiar manner of a galvanometer element, theoscillating signal being derived from an audio frequency source 173.Thus the reflected infrared beam 179 will be caused to swing on and oifthe silver chloride window 177 in relation to the signal, and infraredradiation will enter the gas chamber 170 by way of the silver chloridewindow in greater or lesser amount in dependence on the amplitude of theaudio frequency signal.

The population of molecules in their various energy states is altered bythe incident infrared, and this view is useful in visualizing thephenomenon involved.

When the infrared beam is deflected past the silver chloride window 177so that none of it enters the gas chamber 170, the resonant absorptionof the ammonia gas to the microwave energy of 24,000 megacycles has itsfull value of 0.2 decibel per foot of passage through the gas. When thereflected infrared beam 179 is centered on the Window 177, thisabsorption is reduced. Thus the microwave energy passing through thechamber 170 and into the output wave guide 26 is alternately increasedand reduced in accordance with the signal of the source 178; i. e.,amplitude modulation of the microwaves of the source through the mediumof a molecularly resonant gas is provided.

Fig. 2 shows a system for demodulating microwave energy which may bereceived, for example, after radio transmission, and which bears adesired signal in the form of amplitude modulation. The incomingamplitude-modulated microwaves may be picked up by an antenna 130 andsupplied by way of a wave guide 181 and a mica window 24 into a resonantgas chamber 182 as before. An infrared radiation source 183 is providedin a position such that its rays 184 shine directly into the gas chamber182 by way of a suitable window 1&5, for example of silver chloride. Atthe far end of the chamber 182 another silver chloride window 186 isprovided through which emerges the infrared radiation 184 which has notbeen absorbed by the gas in the chamber 182. The amount of absorption ofinfrared radiation is dependent on the excitation of the gas by theincoming microwave energy; and since the strength of the latter variesin accordance with the signal which is amplitude-modulated thereon, sothe transmitted infrared radiation varies similarly. The transmittedinfrared radiation 184, now modulated in accordance with the signal, maybe applied to a suitable detector 187, for example a bolometerthermistor of the type described in an application of I. A. Becker,Serial No. 602,261, filed September 26, 1946, Patent 2,414,792, January28, 1947. The output of this detector 187 may be amplified as desiredand reproduced in a suitable manner, for example, by a telephonereceiver 188. Thus demodulation or detection of amplitude-modulatedmicrowave radiation is etfected through the medium of the resonantabsorption band of a gas.

What is claimed is:

1. Apparatus for modulating the amplitude of oscillations derived from amicrowave source, which comprises a gas at low pressure characterized bysharp resonance lines of selective absorption of incident microwaveenergy of the frequency of said source in an amount dependent on thenumbers of the molecules of said gas in the various possible energylevels, means for guiding waves of said source through said gas, wherebyenergy of said source is absorbed by said gas, an auxiliary source ofelectro magnetic waves of a higher frequency, means for subject ing saidgas to the field of said auxiliary source to alter the said numbers andso the amount of said absorption, and means for varying the strength ofsaid auxiliary source field under control of a modulating signal.

2. In a signaling system, a body of gas characterized by the sharpmolecular resonance absorption of incident electromagnetic wave energyof one frequency in an amount dependent upon the numbers of themolecules of said gas in the various possible energy levels asdetermined by the energy level of electromagnetic waves of a secondhigher frequency also incident upon said gas, means for impressingelectromagnetic wave energy of said one frequency on said gas, means forimpressing electromagnetic wave energy of said second frequency on saidgas at energy levels varying in accordance with a signal, and meansresponsive to the electromagnetic wave energy of said one frequencyafter transmission through said gas for communicating said signal.

3. A signalling system comprising a generator of microwaves, a hollowwave guide connected thereto, a resonant cavity interposed in said waveguide having therein a gas at pressures sufiiciently low in themillimeter pressure range to provide sharp molecular resonanceabsorption, a source of energy for irradiating said gas to vary theabsorption coefiicient of said gas whereby the amplitude of themicrowaves transmitted through said gas is varied, and means utilizingthe microwaves transmitted through said gas.

4. The system of claim 3, wherein said irradiating source has afrequency higher than said microwaves and means for varying theirradiating energy in accordance with a signal.

5. A signalling system comprising a wave guide, a gas at low pressureconfined in said wave guide to provide sharp molecular resonance lines,a source of microwaves of frequency corresponding to one of saidresonant lines, means for transmitting waves from said source throughsaid wave guide, another source of electromagnetic waves of frequencyhigher than said microwaves, means for irradiating said gas by wavesfrom the second source, means for varying in accordance with a signalthe energy of waves irradiating said gas to vary the absorptioncoefiicient of said gas whereby amplitude modulation of the microwavesis produced.

References Cited in the file of this patent UNITED STATES PATENTS

